Cold vs warm zone hardiness

[NC_Palms]

I am sorry in advance if this comes off as a stupid question.

A lot of times I hear people say "my zone is a cold 8b" or "my zone is a warm 9b" but what differentiates a warm zone from a cold zone? So for example, according to the USDA interactive hardiness map my average minimum temperature is 11.5ºF so I'd imagine that would be considered a cold 8a while Cape Hatteras would be considered a cold 9a with an average minimum temperature of only 20.0ºF? 

 

[enigma99]

I think it’s just like I would say I am in a Zone 9b+ or a warm 9b. Annual lows are around 29 and hardly ever much lower. Although it can dip lower on occasion. But 29 vs 25 (still Zone9b) is huge... if I got down to 25 or 26 every year I couldn’t possibly grow what I grow. So that’s probably why people use terms of cold and warm.

[GottmitAlex]

1 hour ago, NC_Palms said:

I am sorry in advance if this comes off as a stupid question.

A lot of times I hear people say "my zone is a cold 8b" or "my zone is a warm 9b" but what differentiates a warm zone from a cold zone? So for example, according to the USDA interactive hardiness map my average minimum temperature is 11.5ºF so I'd imagine that would be considered a cold 8a while Cape Hatteras would be considered a cold 9a with an average minimum temperature of only 20.0ºF? 

 

A rose by any other name would smell as sweet.

[oasis371]

The gardening zones that are provided for regions throughout much of the USA (Eastern, Northern, Southern and Central) rely on AVERAGE, Winter Minimum temperatures (averaged over a number of years). But they do not include HEAT zone data. So it's possible for a plant to be cold tolerant for an area but not have sufficient heat in the growing season.  For example, Cape Cod, Ma is a solid Zone 7 (based on minimum temps), but is sometimes referred to as a COLD Zone 7 because of a relative lack of Summer heat (which many subtropicals need). Those same Zone 7 subtropical plants in the South would have the Summer heat. To give an even better example, San Francisco is a solid (Eastern Zone 10a/b), but the Summers are considered chilly if not COLD by Eastern standards. So, you have to consider both Winter cold and Summer heat in considering a plant's hardiness to a region. 

Edited February 19, 2019 by oasis371
clarification

[Silas_Sancona]

1 hour ago, oasis371 said:

The gardening zones that are provided for regions throughout much of the USA (Eastern, Northern, Southern and Central) rely on AVERAGE, Winter Minimum temperatures (averaged over a number of years). But they do not include HEAT zone data. So it's possible for a plant to be cold tolerant for an area but not have sufficient heat in the growing season.  For example, Cape Cod, Ma is a solid Zone 7 (based on minimum temps), but is sometimes referred to as a COLD Zone 7 because of a relative lack of Summer heat (which many subtropicals need). Those same Zone 7 subtropical plants in the South would have the Summer heat. To give an even better example, San Francisco is a solid (Eastern Zone 10a/b), but the Summers are considered chilly if not COLD by Eastern standards. So, you have to consider both Winter cold and Summer heat in considering a plant's hardiness to a region. 

Good thoughts, ..The San Francisco reference might be the best example of the difference when looking at the warmer / cooler end of a particular zone.  "Plentiful heat Vs. heat deprived zone X"

Another example i was thinking about would be the differences between say zone 9b here in the low desert vs 9b back where i grew up in San Jose (CA.).. or, for the sake of tropical-esque plants, 9b in Florida or Southern Texas..

We here in Phoenix (..and in Florida) can grow certain stuff more effortlessly than i might be able to in San Jose, yet, there are several things that might be more challenging to grow here vs back in FL., or vise versa,  or back in San Jose.. All three areas might only average 28-29F at the lowest, thus being solid 9b areas... but the heat and almost always dry air here can damage stuff that grows without much issue in Florida.. At the same time, we here in Phoenix can have two rainy seasons with the summer Monsoon season being especially important for stuff that likes heat, humidity, and moisture during the growing season.. Among the two areas, Florida is obviously the best area in regards to receiving summer precip. while any rain, beyond a quick, occasional sprinkle during the summer in San Jose, is quite rare.. Both Florida and San Jose don't typically see many ( any) days of extended summer heat exceeding 100F.. San Jose can experience summer days where effects from the summer marine layer really keep down temperatures, especially night time lows. All of this plays into how easily or not so easily one could grow X plant in their garden..

Overall Cold tolerance plays a big b part of course, but if that were the only factor to be considered, growing something like Licuala peltata or Veitchia would be far less challenging than it can be / has been here. Our cold might not be bad enough to kill either most years, but the heat / sun intensity, and lack of consistent humidity / rain during the summer can easily fry both to death. 

In the referenced case of San Francisco, someone there could grow all sorts of cool stuff that wants a zone 10 climate, but despises *real* heat like what you will experience in zone 10 in Florida.. or here.. 

Even a zone like say zone 6b or 7a in Kansas can subtle exhibit differences that can influence what can be grown easily when compared to the same zone in Washington State, or Washington D.C. 

[mdsonofthesouth]

Cold zone 8b is average low nearer to 15F. Warm zone 8b would be closer to 20F.

 

Thats another way I determine cold vs warm, but the above answers are solid as well. I use their way as well.

Edited February 20, 2019 by mdsonofthesouth

[Dave-Vero]

The American Horticultural Society heat zone map is worth checking.  Trachycarpus palms die of heat death in southern Florida.